Respirators typically include a filter canister having a carbon filter that includes activated carbon granules. The carbon granules serve to remove certain gases and vapours, such as organic chemicals, through either physisorption or chemisorption. The carbon granules are formed as a layer between inlet and outlet surfaces of the filter canister. In use, ambient air is inhaled through the inlet surface which then exits the outlet surface as filtered air to be breathed in by the wearer of the respirator.
In filter canisters having a flat filter bed, the filter bed is filled with carbon granules to form a flat carbon layer wherein the depth of the carbon layer is substantially constant in order to obtain optimum filtering performance. A snowstorm filling technique may be used to distribute the carbon granules in the filter bed. In this technique, carbon is fed relatively slowly, at a controlled rate into a filling tube having screens configured such that carbon granules fall evenly into the filter bed when exiting the tube to form a flatbed carbon filter. Snowstorm filling techniques provide a flatbed carbon filter having maximum packing density and minimum volume. Typically, the volume of a flatbed carbon filter formed by this technique is approximately 15-20% lower than a flatbed carbon filter bed not formed by this technique. Snowstorm filling the granules into the filter is a preferred filling methodology for filling conventional filters generally having parallel planar inlet and outlet faces. In snowstorm filling, carbon granules are dropped into a container through a tube containing a number of wire metal screens. The tube has the same face area as the container to be filled. The screen opening size, vertical spacing and other key geometries are related to the particle size of the adsorbent being filled and the geometry of the filter. The screens randomize and uniformly distribute the particles within the container, resulting in a carbon bed that has a minimum volume. This prevents the creation of localized channels within the carbon bed that would provide detrimental uncontrolled flow channels through the adsorption filter. Snowstorm filling is known to provide a maximum density of carbon granules in the filter, which avoid later settling of the adsorbent in the filters. However, snowstorm filling has been limited to use with planar inlet and outlet faces, i.e., filter beds having a uniform flat depth. The use of snowstorm filing for conformal filters has been ineffective because of the mis-distribution of the carbon granules during fill.
Some respirators utilize conformal filter canisters having curved inlet and outlet surfaces to enable the filter canister to closely follow the curvature of a wearer's face. In this configuration, a bottom surface of the filter bed is dome shaped. The use of snowstorm filling techniques to completely fill this type of filter bed is ineffective as its use results in a carbon layer having a flat top surface, but whose depth or thickness is not constant due to the dome shape of the bottom surface. In particular, the thickness of the carbon layer is smallest in an area corresponding to a center of the dome shaped bottom surface and increases in areas further away from the bottom surface. For this reason snowstorm filling is not used for filling conformal filters.
Other approaches for filling a conformal filter canister include filling the canister with carbon granules and then using vibration techniques, e.g., 20 Hz to about 80 Hz, to vibrate the filter canister until a predetermined packing density is reached. However, such approaches result in a carbon layer whose packing density is lower than that achieved by snowstorm filling techniques. This results in a filter that is lower in performance, for the same amount of carbon material, than a respirator having a flatbed filter configuration. The lower density also is susceptible to degradation with exposure to impact forces after manufacture.